I use a hacked version of dcraw to dump out the RAW data and then massage it in Matlab.

After some further experimenting in Lightroom, I have to say that matching an underexposed RAW at lower ISO to a normally exposed higher ISO one is not just a simple matter of moving the exposure slider. See here:
http://forums.dpreview.com/forums/read.asp?forum=1041&message=37799085
. So as a practical matter, it may well be better to stick with what the camera wants to do (use normal exposure at a pushed ISO) unless you're willing to do some work understanding the details of how to get hues and so forth to match. The commercial RAW converters seem to be set up to make the normal-exposure work flow easy (as they should).